To improve thermal efficiency of gasoline internal combustion engines, dilute combustion—using either air or re-circulated exhaust gas—is known to give enhanced thermal efficiency and low NOx emissions. However, there is a limit at which an engine can be operated with a diluted mixture because of misfire and combustion instability as a result of a slow burn. Known methods to extend the dilution limit include 1) improving ignitability of the mixture by enhancing ignition and fuel preparation, 2) increasing the flame speed by introducing charge motion and turbulence, and 3) operating the engine under controlled auto-ignition combustion.
The controlled auto-ignition process is sometimes called the Homogeneous Charge Compression Ignition (HCCI) process. In this process, a mixture of combusted gases, air and fuel is created and auto-ignition is initiated simultaneously from many ignition sites within the mixture during compression, resulting in very stable power output and high thermal efficiency. Since the combustion is highly diluted and uniformly distributed throughout the charge, the burned gas temperature, and hence NOx emission, is substantially lower than that of the traditional spark ignition engine based on propagating flame front, and the diesel engine based on an attached diffusion flame. In both spark ignition and diesel engines, the burned gas temperature is highly heterogeneous within the mixture with very high local temperature creating high NOx emissions.
Engines operating under controlled auto-ignition combustion have been successfully demonstrated in two-stroke gasoline engines using a conventional compression ratio. It is believed that the high proportion of burned gases remaining from the previous cycle, i.e. the residual content, within the two-stroke engine combustion chamber is responsible for providing the high mixture temperature necessary to promote auto-ignition in a highly diluted mixture. In four-stroke engines with traditional valve means, the residual content is low, controlled auto-ignition at part load is difficult to achieve. Known methods to induce controlled auto-ignition at part load include: 1) intake air heating, 2) variable compression ratio, and 3) blending gasoline with fuel that has wider auto-ignition ranges than gasoline. In all the above methods, the range of engine speeds and loads in which controlled auto-ignition combustion can be achieved is relatively narrow.
Engines operating under controlled auto-ignition combustion have been demonstrated in four-stroke gasoline engines using variable valve actuation to obtain the necessary conditions for auto-ignition in a highly diluted mixture. Various fueling controls including split and single injections have been proposed for use in conjunction with valve control strategies to maintain stable auto-ignition combustion across a variety of engine load conditions.
In commonly assigned U.S. patent application Ser. No. 10/899,457 an exemplary fuel injection and valve strategy for stable, extended controlled auto-ignition is disclosed. Therein, during operation with low part load, a first injection of fixed amount of fuel during the negative valve overlap period is followed by a second fuel injection during the subsequent compression stroke. The injection timing for the first injection retards and the injection timing for the second injection advances in a continuous manner as the engine load increases. During operation with intermediate part load, a first injection of fuel during the negative valve overlap period followed immediately by a second injection of fuel during the subsequent intake stroke supports auto-ignition. Optimal separation of the two injections is around 30 to 60 degrees crank angle. The injection timings of both injections retard in a continuous manner as the engine load increases. And, during operation with high part load, a single fuel injection during the intake stroke supports auto-ignition. The injection timing retards as the engine load increases.
Lean air-fuel ratio operation is the preferred mode from low load to high part loads for best fuel economy. However, as the engine load or fueling rate increases, the engine-out NOx emission also increases. Up to certain engine load, the level of NOx emission can be kept below a limiting value. The NOx aftertreatment conversion efficiency reduces drastically if a traditional three-way after treatment device is used with lean engine operation. A switch from lean to stoichiometric engine operation is therefore necessitated at some point as load increases such that the traditional three-way after treatment device can be used for effective NOx emission control.
Further extension of the mid load operation limit of a gasoline direct-injection controlled auto-ignition combustion engine that is capable of using a conventional three-way after-treatment system as an emission control device can be achieved by employing a fuel injector with multiple injection capability and a spark plug. A first fuel injection occurs during early intake stroke to form a lean air-fuel mixture throughout the combustion chamber near the end of the compression stroke. A second fuel injection occurs in either the later part of the intake stroke or the compression stroke to create a stratified air-fuel mixture with ignitable mixture near the spark plug. The spark plug is used to ignite the ignitable mixture and its timing strongly influences the combustion phasing. The spark-ignition combustion works as an ignition source to trigger the auto-ignition of the surrounding lean mixture to burn at a target crank angle after TDC of the compression stroke. In this way, a mixed-mode combustion process that consists of two separate yet related processes is realized. Further, the engine is operated at the stoichiometric air fuel ratio with external EGR dilution so a traditional after-treatment device is sufficient for engine-out emission control. The external EGR dilution also acts as an effective combustion rate control parameter during the auto-ignition combustion phase. The high load limit of a gasoline direct-injection controlled auto-ignition combustion engine is expanded by more than 10% with acceptable maximum rate of pressure rise or amplitude of pressure oscillation.
While the advances outlined above have successfully demonstrated controlled auto-ignition capabilities at steady state conditions, rapid load changes or transients may introduce undesirable combustion results. Commonly assigned and U.S. patent application Ser. No. 60/658376 describes a system and method for robust auto-ignition combustion control during load transients from low load to high part load. For engine operations with lean air-fuel ratio without external EGR, feed forward control with lookup tables and rate limiters is sufficient to ensure no misfiring and partial-burn during low load to high part load (and vice versa) transitions. However, load transitions between high part load and medium load continue to present challenges with respect to misfiring or partial-burns.